From the earliest period in the Garand we have good evidence that the receiver bridge was never effective in preventing slamfires. The Army lightened the early Garand firing pin, just as they did the M16 firing pin, and the only plausible reason for this is because they were having slamfires. It adds cost to mill out material and makes the midsection of the firing pin susceptible to breakage. Making the 30-06 primer less sensitive would have cured the in battery slamfire issue, but there were other weapons in inventory using 30-06 cartridges. If the Army made the large rifle primer less sensitive, that could have created failures to fire in BAR’s, M1919’s, 03 Springfields. Maybe the Lewis Guns were still in inventory. The path of least resistance was to reduce the kinetic energy of the firing pin by reducing its weight.
The bottom firing pin is from a M1 Carbine. As the first of its type, the Army was able to specify a relatively insensitive primer for the mechanism and thus the firing pin was not scalloped to reduce weight. This is a picture of the rare round Garand firing pin. So rare they sold for $100.00 apiece at Orion7, and they sold out so fast that I was unable to purchase one.
Even with “mil spec” primers, some slamfires must have occurred, though whatever safety reports were generated have not, and will not be released to the public. Safety incident reports are not even shared within the Army, only those Safety and Law Enforcement who have a valid reason are granted access. If you have not seen these reports, it is because you don’t have a need to know. Army leadership decides who has a need to know, it is not your decision regardless of how curious you might be. The Garand mechanism was used by other nations and it is instructive that the Italians added a firing pin spring to the bolts of their Garand based BM 59 rifles. Obviously they had experienced slamfires in their Garands previous to designing this mechanism.
I am of the opinion that the receiver bridge simply holds the back of the bolt up and cams the firing pin out of the primer pocket during extraction. I am dubious as to the effectiveness of the thing in preventing slamfires. Generations of shooters have taught to worship it as a “safety bridge” but I think the whole religion is a lie, was known to be a lie when it was created, and I don’t trust receiver bridges to be anything but a firing pin cam.
The proper primer to use in military weapons is the primer that the Military specified for the weapon. For the Garand, M1a, I can’t say it any better than what is to be found in the Springfield Armory Manual, page 4
www.springfield-armory.com/download.php?asset=M1AManual.pdf
Ammunition
The M1A is designed and built to specifications to shoot standard factory military 7.62 NATO ammunition. The specifications for standard military ammunition include harder primers to withstand the slight indentation from the firing pin when the bolt chambers a cartridge. This slight indentation is normal. The use of civilian ammunition with more sensitive primers or hand loads with commercial primers and/or improperly seated primers increase the risk of primer detonation when the bolt slams forward. This unexpected "slam fire" can occur even if the trigger is not being pulled and if the safety is on. Use of military specification ammunition will help avoid this.
Every shooter should use extreme caution when loading this or any other firearm. See page 17 for instructions on proper loading to help avoid a "slam fire". Also see enclosed article on “Slam Fire” written by Wayne Faatz
It is my opinion, based on research of periodicals, DTIC documents, that in-house Army expertise on all things firearms and ammunition was at its peak till the late 60’s. An aggressive campaign to outsource and shutter Government design and manufacturing facilities was initially started by Secretary of Defense Robert McNamara. Over time the Government lost its in-house cartridge designers, manufacturing facilities, firearms design and manufacturing. Regardless of what they call themselves, the Government no longer has real technical expertise in firearms design, manufacture, or cartridge design and manufacture. The real value added functions have been outsourced and what in-house “technical experts” the Government has are technical Contract Experts. These guys are experts in writing, evaluating, awarding, overseeing contracts. The Government does not design or build anything, it hires Contractors to do all the thinking, designing, and making. Government management oversight is totally woeful and inadequate as many Technical Managers really don’t know what they are doing, often don’t care, and even if they did, don’t have any real authority over their Contractors. Only Congress can correct and cancel a misbehaving contractor. The history of Government Procurement is a long litany of cost, schedule, overruns and of the purchase of equipment that fails once it reaches the field. There was a time when the Army had people who could design a Garand , M14, or a M60 machine gun, and the ammunition to fit, but not anymore. However, when it did come to the Garand, M14, the in-house technical expertise was at its highest, and the 34 primer was the primer selected, because of its characteristics, as the best choice for these mechanisms. Reaching back into history, all you have to do is search DTIC and you will find references, such as on page 4-69 of the
"Report of the M16 Review Panel Appendix 4 Appendix 4 Ammunition Development Program"
You can find this at
http://www.dtic.mil/dtic/index.html.
The Army was having all sorts of issues, slamfires, primer sensitivity, etc, with the early M16’s. The quote from the primer sensitivity section was
“Contrary to the requirements for 7.62 mm ammunition, which specifies that the FA34 primer is mandatory, no attempt has been made to standardize on type of primer for 5.56 mm ammunition”.
A series of nine reports were written in 1959, by Reed Donnard, all of them titled :
“The Standardization of Lead Styphnate primers, Priming mixtures and process” Report # 1 - 9 These reports dealt with the industry/Government joint development venture where Industry and Government developed a non corrosive primer for use in Army/AF/Navy applications. These reports describe tests, compositions, and weapon characteristics. I never found the weapon characteristic report, that would have been interesting as I am sure they measured the kinetic energy of the firing pin impact on a number of weapon systems. The small arms primer that came out of this Government/Industry joint venture has a specific primer cake composition and a specific sensitivity requirement which is matched to the weapons of the period.
This is the top level drawing for the #34 primer:
The small arms primer mix that came out of this effort is the FA 956 mix,
PATR 2700 Encyclopedia of Explosives Vol 8 gives the composition
FA 956
Lead Styphanate 37.7 +/- 5%
Tetracene 4.0 +/- 1%
Barium Nitrate 32.0 +/- 5%
Antimony Sulfide 15.0 +/- 2%
Aluminum Powder 7.0 +/- 1%
PETN 5.0 +/- 1%
Gum Arabic 0.2%
There are plenty of primer compositions for there are many applications for primers other than small arms. This is a list of military priming mixtures, FA 70 is the old corrosive 30 cal primer, FA 90 was the corrosive 50 cal primer. I was able to identify PA101 as a fuse primer composition. All of the compositions to the right of FA90 are more sensitive than rifle primer compositions, so these are probably used on a variety of explosive or propellant devices.
This is a neat chart. The Government is looking for a lead free primer and this chart compares impact energies and ignition probabilities of red phosphors primer mix and the lead styphnate.
Primer compound is mixed wet and kept wet. George Frost makes the claim that primers are packed wet and that once dry, are extremely dangerous. Somewhere in the book he says that sticking your head beyond the safety barrier to look at the primer shakers is tantamount to committing suicide.
There are several DTIC sources which are very educational on the subject of primers. A good one is
“Percussion Primers Design Requirements , Rev A 1976, and there is a later reversion of this same document. Another outstanding, and I mean really outstanding source is the
Encyclopedia of Explosives PATR 2700 1978. This document came out of Picatinny Arsenal. Basically nothing of significant depth and knowledge appears after the 1970’s. I surmise that all the real in-house legacy technical expertise within the Government had retired and after 1980 what you see in the system as “technical reports” are in actuality briefings or reports created by Support Contractors. These are very “fluffy” as the in-house Government Managers don’t know enough to ask deep, relevant, or penetrating questions.
The absolute best reference I have found on primers, and ammunition making, is the NRA book
Ammunition Making by George Frost. I bought mine in 1991 and unfortunately, this book is out of print. This book actually tells you how to make every chemical used in primer mix and how to mix it all up into primer cake. This is not something that I recommend anyone does, but the knowledge of the chemistry and processes is very interesting. On page 60 of the book, Mr Frost shows the average cup thickness of various primers, and the military small rifle primer is a thicker cup. Curiously he does not show the thicknesses of the military large rifle cup. Starting on page 102 is an excellent section on Primer Sensitivity. It is my recollection that a CCI Quality Engineer told me that the #34 primer has an H bar two inches higher than their commercial primer.
You would expect that each lot of primers would have a different sensitivity as some constituents can vary by up to 5%, and then purity of ingredients can vary, and there is also is the issue of how primer cake is made. From those who have seen it, primer cake is a wet slurry mixed by hand, the workers using paddles. I really doubt the homogeneity of the mix is perfectly even and so it makes sense that once in a while a bit of primer cake ends up creating a dud, and once in the while the process spits out an extra sensitive primer.
PATR 2700 gives this information into the design of primers:
The sensitivity of a primer for a given firing pin/weapon system is then designed into the primer by the proper choice of the thickness of the base of the primer cup, the point radius of the anvil, and the degree of compression of the mixture between the anvil point and the cup. This is controlled by the degree to which the anvil is compressed into the cup during manufacture of the primer. In addition, some influence of further compression can be achieved when the primer is inserted into the cartridge case and crimped.
These are all fire and none fire (all misfire) military primer requirements from Mil P 46610. This used to be the test method for determining ignition characteristics of primers, I don’t know if the semi conductor revolution has changed the test method, but basically a ball is dropped on a sample of primers at different heights.
#41 small rifle primer
4 ounce ball
All Fire Height 12 inches
Kinetic Energy 48 in oz
None fire height 3 inches
kinetic energy 12 in oz
#34 large rifle primer
4 ounce ball
All Fire Height 15 inches
Kinetic Energy 59.4 in oz
None fire height 3 inches
kinetic energy 12 in oz
PATR 2700 has this section on the testing of a primer lot:
The actual test procedure generally prescribes a sample of 400 primed cases. Fifty primed cases are tested at each height (where the drop-height is measured as the distance from the bottom of the ball to the top of the firing pin). The number of primers that fire-or fail to fire- is recorded, and this procedure is repeated at successively increasing heights of fifty primed cases at each height until a height is reached at which all test samples in the group being tested fire. The test procedure is repeated at successively decreasing heights of one inch each, using a new group of fifty test cases at each height, until a height is reached at which all primers tested fail to fire.
The raw data of fires and “no fires” is translated into meaningful sensitivity data by use of a calculation sheet shown in Table 1.
The data thus calculated, Ħ, the height at which 50% of the sample fired and ỡ, the standard deviation, can then be used to calculate the height at which all primers will be expected to fire at a chosen confidence level:
Ħ + 5 ỡ = All Fire Height (height at which not more than 3 out of 10,000,000 would misfire)
And also to determine the height at which all will misfire:
Ħ -2 ỡ = All misfire height (Height at which not more than 1 out of 50 will fire)
The none fire, or all misfire fire height allows 1 out of 50 primers to ignite. Extrapolating, out of a case of 5000 primers, you should expect 100 to ignite at the lowest energy level. I don’t consider that particularly comforting. On page 103 of Ammunition Making, George Frost has a table of normal ranges for Ħ depending on the primer application. Still, there are several things to be cautious of when looking at this test procedure and the averages that result. If 400 total primers are tested the lower drop limit is determined by a test of fifty primers. If only one primer goes off, the lot is considered acceptable in so far as the “None-Fire” or All misfire height. To me, fifty primers is not a reassuringly large enough sample to persuade me that an extra, extra sensitive primer might be lurking within the rest of the lot. There is also the assumption that the corporation follows its own procedures. I think everyone is aware that when enough profit is at risk, Corporations will over ride their quality inspectors and ship non conforming material. For corporations it is a monetary calculation: scrapping a lot results in 100% loss of profit, shipping a defective lot will result in some profit loss by customers returning defective product. There maybe potential costs if it goes to court, but since no one sues, the profit loss will be less than scrapping the lot. Given that the ammunition industry is self regulated, there is always some risk that the lot of primers you buy may not exactly be within spec. As a recent example of industry self regulation, take the example of the Herbal Supplement industry . As it turns out, when tested, nearly 80% of supplements were found to contain none of herbal supplement claimed on the label. Most contained garlic, rice, wheat, beans, but no herbs.
http://well.blogs.nytimes.com/2015/02/03/sidebar-whats-in-those-supplements/?_r=0
SAAMI has a primer specification but I don’t have a rifle primer copy so I don’t know the difference in sensitivity between military large primers and commercial. While I can look up the military specifications, SAAMI specifications are hard to find, and then, they are voluntary. There is no legal liability if a company decides they don’t want to follow SAAMI specs and there is no way for us to determine if they are following any criteria. I called a couple of primer manufacturers, CCI was most helpful, Winchester told me everything was proprietary. None of them actually gave me numbers from lot acceptance tests and unless you happen to work at a primer manufacturer, or own the company, you won’t ever know just what is going on in the primer testing facility. This creates an information void. For us on the outside of the industry, monitoring posted slamfire incidents is the only way to evaluate the sensitivity of a primer brand. Manufacturer's are free to change primer characteristics whenever they think the market has moved. As an example, about 1999 Winchester changed over from a nickel plated primer to a brass finish primer. Winchester told me they had made the primer more sensitive to combat off center firing pin hits. They must have made the cups thinner or softer because the brass finish WSR pierced at loads that never bothered the good old nickel plated WSR. Prior to the change, I used to recommend WLR for service rifles but now I don’t. I never heard of slamfires with nickel plated WLR’s. Does not mean they did not, but prior to the internet age, there was a limit to the number of contacts and information sources. Searching the internet, there are reports of slamfires in every mechanism with a free floating firing pin (except roller bolts) and with every primer. The most slamfiring primer of all time is the Federal 210. Prior to the internet age, the only way to find out about slamfires was to personally meet and talk to guys who had blown the back end off Garand receivers using Federal primed cartridges. Because the only allowed causes of slamfires were shooter negligence, after enough judgmental attitudes, these guys rapidly stopped talking about their experiences. All the primer manufacturer’s make military specification primers but the only manufacturer offering their military line to the public is CCI. We should encourage all of them to offer their military product line.
